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Gas Induced Orbital Decay of Binary Systems

Most stars are a member of a binary or even higher order system. In different stellar environments differences in the stellar binary populations have been observed. A team of scientists from the Max-Planck-Institute for Radio Astronomy, Bonn, and Jülich Supercomputing Centre looks at very young binary systems still embedded in their natal gas. Using hydrodynamic simulations on GCS supercomputers, the scientists investigate the influences of this surrounding gas onto the period of such binary systems.

The two stars of the binary system orbit each other (see figure) thereby inducuing a density perturbation (Figure, cut through equatorial plane), which manifests itself as wave with decreasing amplitude as function of the distance to the binary (Figure, radial cut, phi = 0° and phi = 90° components are plotted). Three different cases are investigated: (i) a binary consisting of stars of equal mass, (ii) and one with a mass ratio of 1:2 both on circular orbits, and (iii) the case of eccentric orbits.

A circular binary system with equal stellar masses induces two symmetrical spiral arms in the surrounding gas (first row), whereas for a mass-ratio of 1:2 one of the spiral arms nearly vanishes (second row). The binary system on an elliptical orbit (eccentricity = 0.2) results in an asymmetric acoustic wave (third row).

The energy inserted into wave generation is lost from the binary system. As a result the distance between the binary shrinks. Above simulations are used to determine the reduction of the orbit, which in extreme cases could lead to the merging of the two stars.